What Causes High Idle Speed That Won't Come Down?

[McRosalie]

Common causes of high idle speed that won't come down include: 1. Improper throttle body cleaning leading to stepper motor failure; 2. Faulty coolant temperature sensor causing the ECU to misjudge the engine as operating in cold state, resulting in increased fuel injection and higher RPM; 3. Failed or stuck-open solenoid valve causing intake air volume to exceed ECU standards, resulting in high idle; 4. Worn fuel injectors delivering more fuel than standard, causing high idle. Note that higher idle speed during cold starts is normal - the engine requires higher RPM to reach optimal operating temperature quickly, needing richer air-fuel mixture, which increases both air intake and fuel injection, resulting in elevated RPM.